Abandoned mines have long been a favorite backdrop for movie makers and adventure writers. But they could also be put to practical use in producing sustainable energy, three Canadian researchers argue. The water that floods derelict mines is often warm enough to help heat buildings and produce electricity—and with perhaps a million deserted shafts worldwide, there could still be plenty of treasure down in those depths.

Although mines are expensive to build, they are frequently left to flood once the ore is gone. Sometimes, they pose chronic pollution and safety problems. A better option, as Andrew Hall, John Ashley Scott, and Helen Shang of Laurentian University in Ontario write in Renewable and Sustainable Energy Reviews, is for companies to consider whether their flooded mines might become a continuing source of power rather than remain a problem.

“There have been a number of successful installations worldwide to recover energy from mines,” the trio notes. In essence, the mine becomes a giant geothermal heat pump. Operators can pump warm water out of the mine and heat it a bit more, for instance, and then use it to heat buildings. Some are designed “so that in the summer the direction of flow can be reversed and cooling provided as well.”

In Germany, officials are using mine water to heat and cool a 500-year-old castle—and using an old uranium mine to help produce electricity to meet peak demand. In the U.S., a study of some 1600 abandoned mines found that at least 80 produce water warm enough to deserve a second look. ❧